The one on the bottom often won't need watering except in long droughts, if you've chosen your plants correctly. You're right about still needing to do weeding and other maintenance.
My water bill is 50/mo and my parents' is 300 despite having a similar sized house and twice as many people living in it, and it comes down to them needing to water their lawn while my xeriscaped yard has never been watered. And I'm in a much drier climate.
The most annoying part is that when we do have rainy periods the growth is insane and cause a big pruning/weeding job, but it's otherwise pretty low maintenance. A couple hours ever few weeks to pull some weeds, prune bushes and snip down seedlings.